The present invention relates to stream selection. More particularly, the present invention relates to the selection of sample streams being routed to a process analyzer.
It is common practice to utilize a single automated process analyzer for analyzing multiple sample streams. This signficantly reduces the cost of analyzing gas and liquid process streams in petrochemical plants, refineries and other process-related industries. The sample streams are generally transported near to an analyzer by tubing or piping. An automated valving manifold, usually electronically controlled, sequentially selects and diverts individual sample streams to the automated analyzer. This type of valving arrangement is generally referred to as a "stream-select manifold."
It is extremely important that sample-stream cross-contamination does not occur; i.e., absence of contamination of one sample stream by another in the sample stream selected for analysis. The most likely source of cross-contamination is from leaking "stream-select valves" in the stream-selection valving manifold.
Another common problem is the contamination of a stream selected for analysis by residual fluid from a previous sample stream. This is likely to occur in a common passageway between the valve manifold and the analyzer. Valve-manifold designs have either "dead volume," irregular passageways, or large internal volumes, which require longer period of sample flow (purging) before all residual fluid from a previously selected sample stream is removed. A common source of "difficult-to-purge" internal valve manifold volume is pipe fittings which provide an irregular internal surface. A common source of dead volume is the space between the block valve of a "non-selected" stream and the common sample fluid passage to an analyzer.
When longer sample-stream purge periods are required, it reduces the number of analyses which can be performed by an analyzer in a given period of time. This can increase the cost for analysis by requiring additional analyzers or otherwise negatively impact process adjustments based on current analysis.
Most important, however, is the increased volume of purged sample material which must be discarded. This also increases cost and presents a greater risk of contaminating the environment.
Valves utilized in stream-select manifolds for the most part were designed for ordinary pneumatic and hydraulic fluid-handling applications. Designs have been altered to some degree to accommodate stream-select manifold requirements. But few are designed exclusively for that purpose. Those which are designed primarily for stream selection are of the single block valve design, and are therefore prone to cross-contamination problems when even a slight leak develops.
Modular valve manifold arrangements are well known and in common service. These manifolds, however, are designed to facilitate the addition or removal of individual valves, and to reduce the number of tube and/or pipe fittings required. Their main purpose is to reduce space in pneumatic and hydraulic, not analytical, applications. Hence little consideration has been given to reducing internal volume and/or "dead" (unpurged) space, or the prevention of cross-contamination by residual fluids. Few if any provide double-block-and-bleed (DBB) protection from cross-contamination. Some manifold/valve designs even allow stacking of manifold modules to create a manifold of a desired length; however, the valves are a separate entity, and are attached to the manifold.
Another common problem with stream-selection valves is "fugitive emission" of sample fluids. This typically occurs when a valve stem seal fails. A metal bellows or diaphragm is frequently employed to seal the external actuation linkage to a valve's internal sealing mechanism. This arrangement, particularly when used in combination with a secondary packing, is very effective in reducing fugitive emission from valves. However, embrittlement of metal bellows or diaphragms, particulaly in hydrogen-rich sample-stream service, and fatigue from repeated actuation, often causes premature stem-seal failures. Additionally, valves employing the bellows/diaphragm seal design are expensive, thus limiting their application. Furthermore, when the bellows or diaphragm fails, there can be an abrupt release of potentially flammable and/or toxic fluids to the surrounding environment. In summary, bellows/diaphragm valve stem seals are very effective during their normal service life, but have a severe and potentially unsafe failure mode.
Pneumatic actuation is often preferred in lieu of electric actuation for valves used in hazardous or electrically-classified environments. Current valve designs generally employ discrete pneumatic actuators usually mounted external to the valve with mechanical linkage through a seal to the internal valving mechanism. This arrangement results in a bulky design which takes up large amounts of valuable panel space. This is a particularly important consideration when considering the cost of providing panel space in a typical analyzer housing or environment. The large bulk also precludes close coupling of valves to minimize internal valve-manifold volume.